We are studying the mechanisms whereby inhaled antigen enters the circulation and the influence of inflammation and immunization on these mechanisms. We have examined the absorption of 131I human serum albumin (HSA) through isolated perfused rabbit lungs for 4 hours following aerosol administration. Samples of the blood perfusing the lungs contained 131I in two fractions. One was antigenically intact (about 5 percent of the inhaled dose at 4 hours); the other was a metabolite which co-chromatographed with iodotyrosine (about 5 percent of the inhaled dose). Prior immunization with HSA reduced the quantity of antigenically intact 131I reaching the blood during the study period to about 1.4 percent of the inhaled dose, indicating that immunization inhibited antigen absorption. However, the quantity of the metabolite reaching the blood was increased to about 7 percent of the inhaled dose by immunization. We intend to evaluate the influence of chronic pulmonary granulomatous inflammation on antigen absorption and to determine whether inhibition of absorption by immunization is altered by inflammation. Class of antibodies involved will be determined by passive immunization with specific class deficient antisera.